The subject matter disclosed herein relates to electrical machines. More specifically, the subject matter disclosed herein relates to liquid cooling of magnetic cores of electric machines.
A typical liquid cooled electric machine includes a rotor having a core and one or more rotor windings (conductors) extending therethrough. In some machines, permanent magnet machines, the rotor windings are replaced with a plurality of permanent magnets. The rotor is surrounded by a stator and an air gap exists between the rotor and stator. Similarly, the stator includes a stator core having one or more stator windings extending therethrough. High power density electric machines (either generator or motor) produce intense resistive heating of both the stator and rotor windings and eddy current and magnetic hysteresis heating of the rotor and stator cores. Typical methods of stator cooling include utilizing an end-turn spray and thermal conduction through the back iron to a cooled housing or fluid media. The end turn spray is most often from orifices in the rotor, but it can be supplemented with fixed spray nozzles on the housing. The spray is directed at end turns of the stator windings to cool by impingement. Back iron cooling includes directing cooling liquid through one or more channels in the back iron (housing) radially outboard of the stator core. These cooling methods, however, provide cooling only on the radial and axial periphery of the stator core. Therefore, a hot spot in the stator windings can occur at the axial centerline of the stator core. With physically larger machines the conduction distances from the axial center position of the slots and teeth of the stator core becomes greater, limiting the power level of the machine or requiring lower temperature coolants.
Cooling of wound rotors at low power densities is typically achieved by an oil flow in the rotor shaft and accompanying end turn spray from orifices in the rotor. Cooling of rotors at high power densities is achieved by an oil flow in the rotor shaft and axial flow within the rotor windings.